DESCRIPTION

"Prima::ImageViewer" is a descendant of "Prima::ScrollWidget"
and inherits its document scrolling behavior and programming interface.
See Prima::ScrollWidget for details.

API

Properties

alignment INTEGER

One of the following "ta::XXX" constants:

ta::Left
ta::Center
ta::Right

Selects the horizontal image alignment.

Default value: "ta::Left"

image OBJECT

Selects the image object to be displayed. OBJECT can be
an instance of "Prima::Image", "Prima::Icon", or "Prima::DeviceBitmap" class.

imageFile FILE

Set the image FILE to be loaded and displayed. Is rarely used since does not return
a loading success flag.

quality BOOLEAN

A boolean flag, selecting if the palette of "image" is to be
copied into the widget palette, providing higher visual
quality on paletted displays. See also ``palette'' in Prima::Widget.

Default value: 1

valignment INTEGER

One of the following "ta::XXX" constants:

ta::Top
ta::Middle or ta::Center
ta::Bottom

Selects the vertical image alignment.

NB:"ta::Middle" value is not equal to "ta::Center"'s, however
the both constants produce equal effect here.

Default value: "ta::Bottom"

zoom FLOAT

Selects zoom level for image display. The acceptable value range is between
0.01 and 100. The zoom value is rounded to the closest value divisible by
1/"zoomPrecision". For example, is "zoomPrecision" is 100, the zoom values
will be rounded to the precision of hundredth - to fiftieth and twentieth
fractional values - .02, .04, .05, .06, .08, and 0.1 . When "zoomPrecision"
is 1000, the precision is one thousandth, and so on.

The reason behind this arithmetics is that when image of arbitrary zoom factor
is requested to be displayed, the image sometimes must begin to be drawn from
partial pixel - for example, 10x zoomed image shifted 3 pixels left, must be
displayed so the first image pixel from the left occupies 7 screen pixels, and
the next ones - 10 screen pixels. That means, that the correct image display
routine must ask the system to draw the image at offset -3 screen pixels, where
the first pixel column would correspond to that pixel.

When zoom factor is fractional, the picture is getting more complex. For
example, with zoom factor 12.345, and zero screen offset, first image pixel
begins at 12th screen pixel, the next - 25th ( because of the roundoff ), then
37th etc etc. Also, for example the image is 2000x2000 pixels wide, and is
asked to be drawn so that the image appears shifted 499 screen image pixels
left, beginning to be drawn from ~ 499/12.3456=40.42122 image pixel. Is might
seem that indeed it would be enough to ask system to begin drawing from image
pixel 40, and offset int(0.42122*12.345)=5 screen pixels to the left, however,
that procedure will not account for the correct fixed point roundoff that
accumulates as system scales the image. For zoom factor 12.345 this roundoff
sequence is, as we seen before, (12,25,37,49,62,74,86,99,111,123) for first 10
pixels displayed, that occupy (12,13,12,12,13,12,12,13,12,12) screen pixels.
For pixels starting at 499, this sequence is
(506,519,531,543,556,568,580,593,605,617) offsets or
(13,12,12,13,13,12,12,13,12,12) widths --- note the two subsequent 13s there.
This sequence begins to repeat itself after 200 iterations
(12.345*200=2469.000), which means that in order to achieve correct display
results, the image must be asked to be displayed from image pixel 0 if image's
first pixel on the screen is between 0 and 199 ( or for screen pixels 0-2468),
from image pixel 200 for offsets 200-399, ( screen pixels 2469-4937), and so
on.

Since system internally allocate memory for image scaling, that means that up
to 2*200*min(window_width,image_width)*bytes_per_pixel unneccessary bytes will
be allocated for each image drawing call (2 because the calculations are valid
for both the vertical and horizontal strips), and this can lead to slowdown or
even request failure when image or window dimensions are large. The proposed
solution is to roundoff accepted zoom factors, so these offsets are kept small
- for example, N.25 zoom factors require only max 1/.25=4 extra pixels. When
"zoomPrecision" value is 100, zoom factors are rounded to 0.X2, 0.X4, 0.X5,
0.X6, 0.X8, 0.X0, thus requiring max 50 extra pixels.

NB. If, despite the efforts, the property gets in the way, increase it to
1000 or even 10000, but note that this may lead to problems.

Default value: 100

Methods

on_paint SELF, CANVAS

The "Paint" notification handler is mentioned here for the specific case
of its return value, that is the return value of internal "put_image" call.
For those who might be interested in "put_image" failures, that mostly occur
when trying to draw an image that is too big, the following code might be
useful:

Performs translation of integer pairs integers as (X,Y)-points from widget coordinates
to pixel offset in image coordinates. Takes in account zoom level,
image alignments, and offsets. Returns array of same length as the input.

Useful for determining correspondence, for example, of a mouse event
to a image point.

The reverse function is "point2screen".

point2screen X, Y, [ X, Y, ... ]

Performs translation of integer pairs as (X,Y)-points from image pixel offset
to widget image coordinates. Takes in account zoom level,
image alignments, and offsets. Returns array of same length as the input.

Useful for determining a screen location of an image point.

The reverse function is "screen2point".

watch_load_progress IMAGE

When called, image viewer watches as IMAGE is being loaded ( see ``load'' in Prima::Image )
and displays the progress. As soon as IMAGE begins to load, it replaces the existing "image"
property. Example: